The present invention relates to methods and devices for measuring a pressure, particularly the stationary or quasi-stationary pressure of a fluid utilizing a pressure-charged measuring system conducted in a cylinder, the piston being connected to a load measuring unit.
The use of a measuring piston conducted in a cylinder for measuring a pressure is well known in the art as represented by the teachings of German AS No. 12 06 174. A problem associated with such conventional pressure measuring devices is that the measuring piston exhibits a relatively high surface friction relative to the cylinder thereby decreasing the measurement precision attainable with such devices. Such conventional measuring devices also exhibit relatively leakage losses and are thus utilized only for obtaining coarse measurements.
In practice, however, a significant need exists for extremely precise pressure measuring devices, particularly for measuring the stationary or quasi-stationary pressure of a fluid such as, for example, for determining the mass (also sometimes referred to as the "weight") of the contents of vertical tanks in tank storage facilities for petroleum products. Other uses for precise pressure measuring devices are, by way of example, in wind tunnel testing for monitoring dynamic pressure. Such applications require pressure measuring devices of the highest precision which exhibit an uncomplicated structure and operation.
Heretofore, pressure measuring devices used, for example, for the gravimetric determination of the amount of petroleum product in vertical tanks exhibit, because of the above-described difficulty in conducting such measurements, an overall error quotient in the magnitude of .+-.0.5% in volume and .+-.1% in the value for the mass derived therefrom. In the case of vertical petroleum product tanks, these errors are due, inter alia, to the circuitous route followed by the fluid in the tank to the measuring device, the temperature influence on the petroleum, and mechanical influences on the tank itself. Due to the current high cost of certain petroleum products, errors of the magnitude described above may make a difference, expressed in monetary terms, of 500,000 DM for a single tanker load. The above problems in the industry are described in an article entitled "Ubliche Methoden zur Bestimmung der Menge von Erdolprodukten in Stehtanks" (Standard Methods for Determining the Amount of Petroleum Products in Vertical Tanks), H. Lerch, appearing in the periodical "Schweizer Ingenieur und Architekt", No. 5, 1980, Schweizerischer Ingenieurs- und Architekten-Verein, Verlag der akademischen Technischen Vereine, Zurich.
As described therein, volume determinations may be undertaken by means of identifying the height of the liquid level in the container or of a representative liquid column. As used herein, the term "vertical tank" refers to a cylindrical container consisting of steel having a vertical axis of the type which is generally employed in storage facilities for storing large amounts of petroleum product. Such a tank may have dimensions of approximately 50 m in diameter and a height in the range of 20 through 25 m.
One method for determining the height of the liquid level in such a tank is to lower a weighted tape measure connected to the top or roof of the tank until the weight touches the so-called reference plate in the floor of the tank. The filling height is read on the scale of the measuring tape. The volume is calculated from the cross-sectional surface integral of the tank and the filling height of the tank. Because the liquid contained in the tank, as well as the tank itself, exhibit a not inconsiderable thermal coefficient of expansion, the result obtained from the measurement must be correlated to a reference temperature. Additionally, the tank expands differentially along its vertical dimension as a result of the increasing liquid pressure at different heights above the floor of the tank and thus the cross-sectional area of the tank will differ at different heights.
If the weight of the tank contents is to be determined utilizing conventional measuring methods, the weight must be calculated from the volume and the density of the material contained in the tank. Determining the density of the contained material with a sufficient degree of precision is difficult, particularly in the case of hydrocarbon fluids such as, for example, gasoline, which exhibit a coefficient of volume expansion on the order of 0.1%/K, thereby requiring that different temperature layers within the tank be taken into consideration. As stated above, an error factor of .+-.0.5% in the volume determination and of .+-.1% in the mass determination is standard with most conventional measuring methods and devices. Similar orders of error are also present for volume measurements undertaken with flow meters, and may be even higher under certain conditions due to wear of such devices.
The above sources of measuring error have a particularly disadvantageous effect when, for example, a portion of the total tank volume is removed from or added to an existing amount of liquid in the tank so that the difference or addition of two successive measurements must be calculated in order to determine the new amount of liquid in the tank, whereby the measuring errors add together under the least favorable conditions.
Another known method for determining the level of liquid in a container, and hence the volume or weight of the liquid, is by the use of so-called buoyancy measurement employing a buoyant member with a scale. This measuring method also exhibits problemmatical sources of error particularly in vertical tanks having a floating roof wherein the introduction of the float is difficult and undesirable because of the unavoidable necessity of providing an opening in the floating roof. Similar considerations apply to containers having a fixed top or roof which also require a passage for the float. Additionally, the measuring means must be encapsulated gas tight so as to be explosion-proof and also to prevent the unwanted release of vapors which may constitute a substantial fire hazard. One attempt to overcome these problems is to accommodate the measuring means in a measuring container which is erected separately next to the tank, however, such a separate structure results in a significantly higher cost, and the problem of sealing is not solved. Additionally, changes in the buoyancy characteristics of the float due, for example, to corrosion, may falsify measurements.
It is an object of the present invention to provide a method and apparatus for measuring a pressure, particularly a stationary or quasi-stationary pressure of a fluid, utilizing a pressure-charged measuring piston conducted in a cylinder and connected to a load measuring means, which method and apparatus combine the highest precision with uncomplicated operation.
Another object of the present invention is to provide such a method and apparatus which provide a pressure measurement independent of temperature.
It is a further object of the present invention to provide such a method and apparatus which permit the pressure measurement to be displayed at any selected location, even at a great distance from the tank.
The above objects are inventively achieved in a pressure measuring method and apparatus which utilize a release agent supplied under pressure between the piston and cylinder such that the piston "floats" within the cylinder in contact-free fashion.
When the piston is conducted in this floating manner within the cylinder in accordance with the principles of the present invention, the wall friction between the piston and cylinder is negligibly small, particularly for a measuring piston held immovably in the cylinder by a load measuring means. As a result, only the pressure which is to be measured influences the piston surface and thus, if the piston is supported by a substantially force-free weighing cell utilized as the load measuring means, a measurement output of the highest precision is obtained.
The measuring method and apparatus disclosed and claimed herein substantially eliminates the above measuring errors unavoidably present in conventional devices. In particular, the influence of temperature is eliminated and thus need not be taken into consideration in determining the pressure of a fluid column as the first measurement undertaken for a gravimetric aggregate measurement.
The above result is achieved with relatively low material outlay due to the uncomplicated nature of the structure for the measuring device. In addition, the measuring system is distance-independent with respect to the structure containing the fluid being measured because the measurement is undertaken with a stationary pressure column and therefore no movement and thus no resistances and corresponding pressure changes are encountered in the measuring lines. In order to eliminate these potential problems, the release agent is supplied to the piston and cylinder at a higher pressure than the fluid to be measured.
A high degree of flexibility in the application of the method and apparatus for measuring a pressure disclosed and claimed herein is achieved because the fluid generating the pressure may be simultaneously employed as the release agent. Alternatively, a different fluid, in the form of a liquid or gas (preferably air) may be utilized. If the fluid generating the pressure to be measured is utilized as the release agent, the pressure of that fluid is increased before introduction into the cylinder as the release agent.
If a liquid is utilized as the release agent, such a liquid should have a higher specific gravity than the fluid to be measured, should be immiscible with the fluid generating the pressure to be measured, and should be non-combustible so as to not be an explosion hazard. By way of example, the contents of a benzene tank could be measured utilizing water as the release agent. If a separating vessel is utilized providing a boundary preventing contact between the two different fluids is utilized, the difference in the specific gravities of the fluids can be neglected because it is only necessary to identify volume differences in the fluid generating the pressure to be measured.
The head of pressure for the release agent is dependent upon a series of factors and is therefore not numerically fixed. The release agent simultaneously performs the two functions of sealing the measuring chamber in the cylinder, that is, the volume which is charged with the pressure to be measured between the cylinder and the piston, against loss of pressure and maintains the gap between the piston and cylinder walls in the form of a lubricant layer so that contact of the walls does not occur. The head of the release agent so confined predominantly depends upon its kinetic viscosity, which may widely vary for different gases and liquids, and further depends upon the surface size and height of the gap and, to a slight degree, on the temperature of the system.
As stated above, the pressure of the release agent must be higher than the pressure of the fluid to be measured which in practice means that the pressure of the release agent must be at least high enough such that the confining function relative to the measured fluid and the contact free piston/cylinder condition are both achieved.
As a result of the extremely high precision obtainable with the method and apparatus disclosed and claimed herein, the method and apparatus may be particularly advantageously employed in the gravimetric determination of the amount of petroleum product in a stationary tank, such as a bulk storage tank. The error factor which is present in conventional methods and devices for undertaking such a gravimetric measurement is reduced by at least a factor of 10 to approximately .+-.0.1%. Additionally, errors present in standard devices which compound when aggregate measurements are combined are substantially eliminated, as are errors due to the influence of temperature or the circuitous route over which the liquid in such standard devices must travel in order to be measured.
In one embodiment of the method and apparatus, the pressure to be measured is transmitted directly to the piston from the vessel containing the fluid. In this embodiment, the line from the tank to the measuring piston is filled with the fluid to be measured. In some instances, particularly wherein a longer line arrangement with a network of measuring lines filled with fluid may be undesirable for safety reasons, the measurement may be undertaken by transmitting the pressure to be measured onto the piston from the fluid by means of a gas, such as air.
The piston for the pressure measuring device disclosed and claimed herein may have a plurality of grooves or pockets in its exterior wall in order to permit hydrostatic or pneumatic accumulation of the release agent thereby improving lubrication. Such pockets may also be disposed in the interior wall of the cylinder and are connected to the release agent supply system.
The measuring piston, as a result of being supported by the measuring unit, forms an essentially force-free weighing system in combination with the measuring unit in which the influence of friction due to dynamic forces of the release agent is negligibly small (practically 0) when such pockets are utilized. Any frictional influence between the piston and cylinder is substantially eliminated, at least in the range of measureable magnitudes.
If a liquid is utilized as the release agent, the cylinder may have a floor including a collecting trough for the release agent and a return line to the release agent supply system for recycling and reusing the release agent. A central opening is provided in the floor of the cylinder for the contact-free passage of a support element connecting the measuring piston to the measuring means.
In the application of the method and apparatus disclosed and claimed herein for the gravimetric identification of the amount of petroleum product in a stationary tank, such as a bulk storage tank, the measured value generator may be connected to a computer to which further parameters such as the size of the effective piston surface of the measuring piston, the integral of the effective cross-sectional area of the tank, and the change of such cross-sectional area under the influence of temperature, as well as weight forces of the liquid, may be supplied for correcting the initial determination of the gravimetric tank contents represented by the signal from the measured value generator.
A network of measuring lines may be provided for a tank storage facility having a plurality of tanks. The network may be equipped with a plurality of switches or valves for connection and disconnection of individual tanks with the measuring apparatus. This arrangement has the advantage that a plurality of tanks can be alternately connected to the measuring means. This permits not only tanks containing the same material but also tanks containing different materials to be monitored if the pressure of the material within the respective tanks is transmitted to the piston indirectly by means of a gas or, if direct transmission of the liquid pressure to the piston is utilized, an additional means may be provided for blowing out the measuring lines after measurement of the fluid in one tank before proceeding to measure a different fluid in another tank.
When measuring a fluid pressure, particularly for combustible fluids, most safety regulations require a hermetic separation between the fluid to be measured and the release agent contained in the system of the measuring device. In a further embodiment of the invention, therefore, such a hermetic separation is provided between the fluid to be measured and the release agent such that those two materials do not come into contact. This hermetic separation does not negatively influence the measuring precision obtained with the method and apparatus disclosed herein thus permitting safe measurement of highly volutile fluids to be undertaken. The separation may be achieved by the use of a resilient or expandable separating element, such as a bellows. In the use of such a resilient separating member, it is essential for precise measurements that the pressure of the release agent be matched to the pressure of the fluid to be measured such that an equilibrium of forces is present on the surfaces of the separating element.
In a further embodiment of the invention, the resilient separating element is disposed in the measuring device such that only one side of the separating element is connected to a feed line for the fluid to be measured, and the other side of the separating element is in connection with a feed line for the release agent. The separating element is held in a neutral position given equilibrium forces and is connected to an indicator means for monitoring the neutral position. The pressure of the release agent normally opposes a change in position of the separating member caused by a change in the pressure of the fluid to be measured.